US 2008/0127635 A1 discloses an exhaust-gas aftertreatment system with a housing in which a plurality of air paths produced by means of a dividing element are arranged. At least one catalyst and one particle filter are arranged in the housing. The at least one dividing element is arranged in a manner so as to result in the formation of two chambers which are located one above the other and are connected opposite the exhaust-gas inlet such that the exhaust-gas stream from the one chamber is deflected into the other chamber. This is to provide an exhaust-gas cleaning apparatus which is reduced in its longitudinal extent, since the two chambers are arranged lying one above the other. The exhaust-gas stream therefore flows successively through the cleaning elements arranged in the housing, said cleaning elements being connected virtually in a row.
WO2006/021337 A1 is concerned with a catalytically coated particle filter with a first and second end surface and an axial length. Starting from its first end surface, the particle filter is coated over part of its length with a first catalyst and, following the latter, with a second catalyst. The first catalyst has platinum and palladium on the first carrier material, whereas the second catalyst contains platinum and, if appropriate, palladium on second carrier materials. To this extent, the particle filter has two catalyst coatings lying in a row with respect to the exhaust-gas stream. Filters of this type have a high thermal mass and only heat up slowly, for which reason an increased concentration of noble metal is provided in the input region of the filter.
WO 2006/021338 A1 discloses a method for coating a wall flow filter. Wall flow filters have two end surfaces and a multiplicity of flow channels running parallel to the cylinder axis. In order to form the filtering effect, the flow channels are closed in an alternating manner at the first and second end surfaces. During its passage through the filter, the exhaust gas has to change over from the entry channels into the exit channels of the filter through the channel walls between the entry and exit channels.
The document 602 22 826 T2 (equivalent to part of WO03/068362) discloses a filter for the aftertreatment of exhaust gas. The filter has a plurality of flow channels which run axially and are closed in an alternating manner at least in a second filter section. The cylindrical filter is a filter roll of folded filter material which is coiled spirally from a web. In a first through-flow section, the exhaust gas flows through without being filtered. The first through-flow section is a central, inner section which is surrounded by the second filtering section of annular design. The central, inner section is merely a through-flow section with open flow channels. The filter section has a catalyst section and a particle filter section which are arranged in a manner such that they follow each other. In one configuration, one part of the exhaust-gas stream flows through the filter section and another part flows through the inner section without being filtered. However, as a result, some of the exhaust gases are not cleaned at all. In order to subject the entire exhaust-gas stream to a cleaning operation, an exhaust pipe is connected to the inner through-flow section such that the entire exhaust-gas stream flows through the through-flow section and enters a rear chamber without being filtered. In said chamber, the exhaust-gas stream is forced to flow back to the inlet side through the filter section in which the catalyst section and the filter section are arranged in a manner following each other similar to the embodiment of WO 2006/021338 A1.
In order to treat exhaust gases from an internal combustion engine, in particular a diesel engine, it is therefore known to arrange a catalyst element and a filter element, in particular a particle filter, in an exhaust system of the internal combustion engine. In this connection, the catalyst element is either arranged upstream of the particle filter or downstream of the particle filter, with it being possible for both components to be arranged in a common housing. Further, gas flow directed between the catalyst and particulate filter may also be included in the single common housing. Of course, the two components can also be arranged in a manner such that they follow each other in separate housings in the exhaust system.
If the particle filter is arranged upstream of the catalyst element, the combustion of soot (regeneration) can be carried out more rapidly, since the exhaust-gas stream still contains a sufficient amount of nitrogen oxide. However, it is disadvantageous that, due to the large thermal mass of the (diesel) particle filter, the temperature in the catalyst element rises very slowly, which results in a reduced conversion of nitrogen oxide by the catalyst. On the other hand, the catalyst or the catalyst element reaches its operating temperature more rapidly if it is arranged upstream of the (diesel) particle filter, which results in greater conversion of nitrogen oxide. However, due to reduced concentrations of nitrogen oxide in the exhaust gas, this results in a reduced combustion of soot, both during active and passive regeneration. Both combinations therefore have as many advantages as disadvantages.
The description is therefore based on the object of improving an exhaust-gas aftertreatment system of the type mentioned at the beginning using simple means in such a manner that exhaust gases, in particular diesel exhaust gases, can be cleaned more efficiently if a catalyst element and a filter element are combined with each other in an exhaust system.
According to the description, the object is achieved by an exhaust-gas aftertreatment system with the features of claim 1, wherein the inlet pipe is divided into a filter pipe and into a catalyst pipe in such a manner that one part of the entire exhaust-gas stream flows through the filter element and another part of the entire exhaust-gas stream flows through the catalyst element, wherein one exhaust-gas part-stream of the exhaust-gas stream emerging in each case from the filter element and the catalyst element is returned in each case via a return pipe, which is arranged in each case on the output side, to the input side of the respective other exhaust-gas aftertreatment element, and wherein the respective other part of the exhaust-gas stream emerging in each case on the outlet side from the filter element and the catalyst element is conducted into the outlet pipe of the exhaust system.
With the description, catalysts or catalyst elements and filter elements are therefore advantageously arranged in such a manner that one part of the entire exhaust-gas stream, i.e. a first part of the entire exhaust-gas stream, is forced, on the one hand, to flow through the catalyst element. Consequently, the other part of the entire exhaust-gas stream, i.e. a second part of the entire exhaust-gas stream, is forced, on the other hand, i.e. at the same time, to flow through the filter element parallel to the first part-stream, with respect to a main direction of flow. In this respect, a virtually parallel connection of the exhaust-gas aftertreatment elements is initially achieved, in which each exhaust-gas aftertreatment element treats one part of the entire exhaust-gas stream.
On the output side of the respective exhaust-gas aftertreatment device, the return pipe is in each case provided firstly as a filter return pipe and secondly as a catalyst return pipe. The filter return pipe returns exhaust gases emerging from the catalyst element to the input side of the filter element. The catalyst return pipe analogously returns the exhaust gases emerging from the filter element to the input side of the catalyst element.
The respective return pipe thus results virtually in an alternating series connection of the two exhaust-gas aftertreatment elements, with the catalyst element firstly being arranged upstream of the filter element for the partial quantity of emerging exhaust gas returned from the catalyst, and the filter element secondly being arranged upstream of the catalyst element for the partial quantity of emerging exhaust gas returned from the filter element.
In order to divide the entire exhaust-gas stream, the inlet pipe has a correspondingly designed component, preferably configured as a Y pipe. As a result, one part of the entire exhaust-gas stream flows in the direction of the filter element whereas the other part flows in the direction of the catalyst element. The respective branches (filter pipe, catalyst pipe) of the dividing component are preferably dimensioned in such a manner that half of the entire exhaust-gas stream is in each case supplied through the respective branch in a manner flowing to the input side of the respective exhaust-gas aftertreatment element. Advantageously, the size of the respective exhaust-gas aftertreatment elements is therefore preferably identical. It is expediently provided that a connecting side of the respective branch, which side opens into the respective inlet side of the respective exhaust-gas aftertreatment element, in each case occupies half of the respective inlet side. It is also possible to design the passages of the respective branches of the dividing component so as to differ such that one exhaust-gas aftertreatment element is supplied with a greater partial quantity of the entire exhaust-gas stream than the other exhaust-gas aftertreatment element. It is furthermore conceivable to arrange a control element in the region of the branch such that, even if the passage of the respective branch is identical in each case, a different part-stream quantity of the divided entire exhaust-gas stream can be achieved in each case. Of course, it is also possible to design the dimensions of the respective exhaust-gas aftertreatment element so as to differ.
On the output side, the respective exhaust-gas aftertreatment element in each case has the connecting element with the outlet pipe and the respective return pipe. In this case, the connecting element of the catalyst element is preferably designed as a T piece into which the connecting element of the filter element opens. Of course, the connecting elements can also have different configurations in order to provide an expedient flow path. The respective return line is designed with its connecting end arranged on the outlet side of the respective exhaust-gas aftertreatment element in such a manner that the exhaust-gas part-stream entering the respective exhaust-gas aftertreatment element (from the internal combustion engine) flows in each case into the respective return pipe. It is advantageous within the context of the description if at least the respective connecting end of the respective return pipe is designed such that it correspondingly matches the attachment size of the catalyst pipe or of the filter pipe.
In a particularly favorable embodiment, it is provided that the filter pipe is attached to the inlet side of the filter element directly opposite the catalyst return pipe, as seen in the axial direction. Analogously, the catalyst pipe is attached to the inlet side of the catalyst element directly opposite the filter return pipe, as seen in the axial direction. The filter return pipe is attached on the input side to the filter element directly opposite the connecting element to the output pipe, as seen in the axial direction. Analogously, the catalyst return pipe is attached on the input side of the catalyst element to the catalyst element directly opposite the connecting element to the outlet pipe, as seen in the axial direction. In this respect, the respective attachments to the respective exhaust-gas aftertreatment element are advantageously arranged in an alternating manner and are designed such that they are correspondingly matched in size. This means that the exhaust-gas part-streams flowing in each case (from the internal combustion engine) first of all flow through the respective exhaust-gas aftertreatment element. The respective exhaust-gas part-streams then enter the respective return line, flow through the respective other exhaust-gas aftertreatment element which is virtually connected in series, and enter via the respective connecting elements into the outlet pipe in which the previously divided exhaust-gas part-streams are mixed together again. It is therefore possible to process the entire exhaust-gas stream.
It is expedient within the context of the description if the filter element is additionally supplied on the input side with hydrocarbons, such as, for example, fuel in order, for example, to increase the temperature of the exhaust gas entering the filter element in each case. However, in particular in the region of the filter return pipe, this may, of course, also be provided in the region of the filter pipe. Of course, it is also conceivable if the catalyst element is supplied on the input side in each case with compounds, such as, for example, ammonia. This can also be provided in particular in the region of the catalyst return pipe, but, of course, also in the region of the catalyst pipe.
The respective return pipes can be matched in their course from the respective outlet side of the respective exhaust-gas aftertreatment element to the respective inlet side of the respective other exhaust-gas aftertreatment element to the construction space conditions, with, of course, a short flow path being desirable. Nevertheless, since the exhaust-gas part-streams which emerge from the respective exhaust-gas aftertreatment elements and are returned have a certain temperature, it can be provided to arrange the return pipes in such a manner that heat can be exchanged between the return pipes and the respective exhaust-gas aftertreatment elements. For example, the return pipes can each be arranged on a housing of the respective exhaust-gas aftertreatment element such that the latter achieves its operating temperature more rapidly by means of transfer of heat.
The catalyst element is expediently designed as an SCR catalyst (selective catalytic reduction), with the filter element being designed as a (diesel) particle filter, for which reason the description is particularly suitable in the case of diesel engines.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.